1. Field of the Invention
This invention relates to seismic sources used in marine operations, and more specifically to systems for deploying such sources.
2. Discussion of the Prior Art
To obtain information on substrata located below a body of water, seismic sources adapted for generating an acoustic signal in the water are towed through the water by towing vessels. The acoustic signal generated in the water travels in all directions, and part of this acoustic energy, after having been reflected or refracted by the substrata, returns to the body of water overlying the substrata, and is detected by the hydrophones of a detector cable that is towed through the water in the neighborhood of the seismic source.
The marine sources most commonly used today are impulsive sources such as air guns. See, for example, U.S. Pat. No. 3,638,752, issued Feb. 1, 1972 to Wakefield. Another type of impulsive source is the water gun. Such sources generate not only the primary pulse, but also unwanted pulses commonly called "ghost" pulses and bubble pulses. These pulses produce unwanted components in the seismic signal transmitted into the earth. The most commonly used marine source today is the air gun and the discussion hereafter will generally refer to air guns, although the principles are not limited to air guns.
Initially, the acoustic wave from a marine source element travels along a spherical wave front. However, when the wave front traveling in an upward direction reaches the water surface, the large velocity discontinuity at the air-water interface causes a "ghost" reflection, which then follows the primary pulse into the substrata.
Air guns, which generate an acoustic wave by the sudden release of a compressed gas into the water, also generate a train of bubble pulses. When the compressed gas is released from the air gun, a gas bubble expands outwardly from the source until the pressure inside the bubble reduces to the point that the hydrostatic pressure of the water causes the bubble to contract. This contraction of the air bubble increases the air pressure within the air bubble again until the internal air bubble pressure exceeds the hydrostatic pressure and the bubble again expands, thereby causing a secondary or bubble pulse. Normally, a series of these bubble pulses will be emitted.
Use of a single source results in a seismic signal whose frequency spectrum exhibits a series of peaks and notches related to the bubble pulse oscillation period. It is a common practice in the industry to utilize an array of air guns of different sizes with different oscillation periods in order to produce a seismic signal having a flatter frequency spectrum. It is also known that the spacing between the air guns can be chosen so as to modify the individual bubble pulse oscillation periods.
Even after the array is "tuned" by the appropriate selection of air gun sizes and spacing between the air guns, the frequency spectrum retains a strong ghost notch. The ghost notch results from the interference between the primary downgoing pulse and the secondary pulse (ghost pulse) which is the reflection of the primary pulse from the water-air interface. The phase differences between the two pulses causes attenuation of spectral components within the bandwidth of the source signatures. Attenuation is most severe at the frequencies where the two pulses are 180.degree. out of phase. At normal incidence, the fundamental ghost frequency for a given source depth can be calculated from EQU fg=Vw/2d=1/.DELTA.tg
where
fg=ghost notch frequency PA1 Vw=compressional wave velocity of the water PA1 d=source depth PA1 .DELTA.tg=time delay between the two pulses.
At incidence angles other than normal, the time delay between the two pulses is not only a function of source depth and water velocity but also of the angle at which the primary pulse is reflected at the water-air interface. As this reflection angle increases, the ghost notch frequency increases.
Normally, the deconvolution process is relied on to remove the effects of the source signature characteristics from the recorded seismic data. Deconvolution methods rely on an operator normally designed from a signature measured directly below the source. The signature is the recording of the pressure amplitude, as a function of time, of the pressure wave generated by a source. However, since marine seismic data are recorded at receiver positions with lateral offsets from the source ranging from several hundred to several thousand meters, the influence of the source ghost will be different at each receiver location because of the change in the surface reflection angle. This situation can produce instabilities in the deconvolution process.
Further, the amplitude of the ghost pulse is dependent upon the magnitude of the reflection coefficient of the water-air interface. Typically a value of -1.0 is assumed for this parameter. However, this value may be a function of the surface wave height and period. Variations in the seismic signal transmitted into the earth due to changes in the water-air surface reflection coefficient can seriously affect the results of deconvolution processes employing an average signature.
All of the difficulties generated by the presence of a source ghost can be remedied if the source can be configured to suppress the generation of the ghost pulse.
United Kingdom Pat. No. 1,193,507, Cholet et al, published June 3, 1970, discloses the placing of a plurality of explosive sources at different depths and producing an emission of sound waves from each source successively at time intervals so that waves propagated downwardly are additive, and waves propagated toward the water surface tend to neutralize. Such an arrangement reduces the ghost pulse.
In the paper presented at the 1984 Annual meeting of the SEG, "Three-Dimensional Air Gun Arrays" by G. C. Smith, there is disclosed a system of four subarrays deployed at different depths and fired at different times. These subarrays are positioned 15 meters and 37.5 meters on each side of a center line, making the four element array 75 meters wide.
U.K. Patent Application No. 2148503A, published May 30, 1985, shows the use of a plurality of implosive sources at different depths whose firing times are controlled to decrease the effect of the reflection of the wave field from the air-sea interface.
The prior art does not show, however, a system for deploying a source array having a plurality of source elements at each of a plurality of depths and in which the source elements are all suspended from a single float to produce an acoustic signal which is tuned to optimize suppression of both ghost effects and residual bubble pulses.
Deploying such an array by means of a single float makes it easier to maintain both the vertical and horizontal distances between all source elements substantially constant. Maintaining constant distances between all source elements is important because the seismic signal generated by the array will change if the spacing between the elements changes. In addition, directive extended source arrays may be formed from a plurality of identical nondirective subarrays without changing the signature of the downgoing wave.